Angewandte Reviews Chemie

How to cite: Angew.Chem. Int. Ed. 2020, 59,15402–15423 PlasticRecycling International Edition: doi.org/10.1002/anie.201915651 German Edition: doi.org/10.1002/ange.201915651 Beyond Mechanical :Giving NewLife to Plastic Ina Vollmer,Michael J. F. Jenks,Mark C. P. Roelands,Robin J. White, Toon van Harmelen, Paul de Wild, GerardP.van der Laan, Florian Meirer, JosT.F.Keurentjes,and Bert M. Weckhuysen*

Keywords: catalysis ·chemical recycling · circularity ·plastic waste · solvolysis

Angewandte Chemie

15402 www.angewandte.org  2020 The Authors. Published by Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim Angew.Chem. Int. Ed. 2020, 59,15402 –15423 Angewandte Reviews Chemie

Increasing the stream of recycled plastic necessitates an approach From the Contents beyond the traditional recycling via melting and re-extrusion. Various chemical recycling processes have great potential to enhance recycling 1. General Introduction 15403 rates.Inthis Review,asummary of the various chemical recycling 2. Problems and Novel Solutions 15410 routes and assessment via life-cycle analysis is complemented by an extensive list of processes developed by companies active in chemical 3. Concluding Remarks and recycling. We showthat each of the currently available processes is Outlook 15417 applicable for specific plastic waste streams.Thus,only acombination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams,while collection and sorting infrastructure will McKinsey for example states that need to be improved, that is,bystricter regulation. This Review aims to “plastics and recycling could generate profit-pool growth of as inspire both science and innovation for the production of higher value muchas$60 billion for the petrochem- and quality products from suitable for reuse or icals and plastics sector”.[2] valorization to create the necessary economic and environmental push While plastic prices were low and for acircular economy. economic incentives for recyclingwere lacking in the past years,China has recently begun rejecting plastic waste from abroad and it is expected that 1. General Introduction many other countries like Malaysia will follow soon. Regard- ing current legislative encouragement for recycling,alegally 1.1. Introduction binding directive of the European Union (EU) states that all plastic packaging shall be recyclable in acost-effective An increase in plastic use (e.g. in everyday products) has manner or reusable by 2030 and aims at making recycling assisted in the rapid economic growth of many economies profitable for businesses.[5] Such legal drivers will push over the last decades.This is in part due to the fact that plastic governmental bodies as well as industry to address plastic materials are typically lightweight, have tunable properties and can easily be shaped, thus lending them to awide range of applications (e.g.automotive,packaging,and housing). How- [*] Dr.I.Vollmer,M.J.F.Jenks, Dr.F.Meirer,Prof. B. M. Weckhuysen Inorganic Chemistry and Catalysis, Debye Institute for Nanomate- ever,durability makes plastics an increasing problem for the rials Science, Utrecht University environment. In contrast to various forms of biomass,such as Universiteitsweg 99, 3584 CG Utrecht (The Netherlands) lignin and chitin, plastic cannot easily be broken down by E-mail:[email protected] microorganisms and leads to environmental pollution. Envi- Dr.M.C.P.Roelands ronmental problems are caused by large plastic pieces when The Netherlands Organisation for Applied Scientific Research (TNO) ingested by animals as well as by micro- and nanoplastics that Delft (The Netherlands) have an uncertain impact on human and ecosystem health and Dr.R.J.White are present in all our surroundings.[1] Thereplacement of The Netherlands Organisation for Applied Scientific Research (TNO) plastics will not necessarily have apositive impact on the Materials Solutions Department Eindhoven (The Netherlands) environment as alternative packaging materials,such as glass or metal containers,are much heavier and increase CO Dr.T.van Harmelen,Dr. G. P. van der Laan 2 The Netherlands Organisation for Applied Scientific Research (TNO) emissions during transport. The260 megatons of plastic Climate, Air & Sustainability Department [2,3] waste produced annually are detrimental for ecosystems Utrecht (The Netherlands) when released to the environment and contributes to climate Dr.P.deWild change because CO2 is emitted during incineration. Themass Energieonderzoek Centrum Nederland (ECN)- part of TNO, Biomass of CO2 emissions amount to three times the mass of plastic & Energy Efficiency incinerated. Hence,plastic waste embodies potential CO2 Petten (The Netherlands) emissions equal to 2% of current global emissions (37.5 giga- Prof. J. T. F. Keurentjes [4] UniversityofTwente, Department of Energy Innovation tons CO2 in 2018). However,plastics also present agreat opportunity if the economic value of all these materials can be Enschede (The Netherlands) maintained. Supportinginformation and the ORCID identification number(s) for This point is also relevant in the context of fossil-fuel the author(s) of this article can be found under: https://doi.org/10.1002/anie.201915651. viability going forward of which 6% are currently used to  2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. produce plastic.[3] This number is expected to increase KGaA. This is an open access article under the terms of the Creative strongly due to decarbonization of the energy and mobility Commons AttributionLicense, which permits use, distribution and sector,while the demand for plastics is increasing due to reproduction in any medium, provided the original work is properly increasing wealth and urbanization globally.A2018 report by cited.

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recycling, ideally in amanner which is circular and enables circulation. It is clear that aconcerted effort from the entire value creation. Other initiatives are ongoing, including plastic industry is required to achieve (e.g. EU) plastic national and European Plastic Pacts,Alliance to End Plastic recycling targets,including polymer manufacturers,recyclers, Waste and New Plastic Economy by the Ellen MacArthur legislators,researchers and consumers. foundation calling for reduce,increased reuse and recycling to tackle the global challenges and moving towards circular plastics economy. 1.3. Scope of the Review As mechanical recycling is typically accompanied by degraded plastic properties,alternative ways to recycle plastic This Review highlights more recent innovations and need to be found. Many reports have highlighted approaches unconventional ideas,which could help solve some of the to the conversion of plastic waste to produce syngas or fuels outstanding issues or even replace existing processes that and naphtha.[2] This Review does not cover gasification routes already received more research focus.Wealso provide an leading to syngas,but mainly focuses on (catalytic) pyrolysis assessment where research attention is needed. This is towards the production of monomers and oligomers as well as performed in three ways.Toset the scene,avision of acircular solvolytic ways to obtain them. This is with the aim of plastic production process chain is proposed based on acircular economy in mind, in which monomers and utilizing waste plastic instead of fossil carbon feedstocks oligomers can be re-polymerized following purification and (Section 1.4.1). Over 150 start-up companies and large plastic used in the same or similar applications as the virgin polymer manufacturers who are already working on components of equivalent, which is produced from fossil fuels.Dissolution/ this vision are highlighted. We explain how these companies precipitation and techniques as well as emerging could make this vision areality and where more effort will be chemical recyclingtechnologies are also presented as such needed. We conducted interviews with selected start-ups to approaches are crucial in moving towards acircular economy discuss experienced barriers to technical implementation as for plastics,supporting an increase above the current 12 wt% well as the need for policy frameworks.Secondly,the plastics/ of globally recycled plastic,whilst also providing solutions polymers streams that have the highest potential to be where mechanical recycling is not possible,such as for foils, adopted in closed loop or high value chemical recycling contaminated and mixed plastic waste streams as well as schemes are analyzed with regard to environmental and multilayer packaging products.[3] economic impacts through an LCA (Section 1.4.2). This Section establishes aranking of available recycling technol-

ogies,interms of their potential for CO2 emission reduction 1.2. Overview of Published Review Articles compared to landfilling and incineration as well as inciner- ation with energy recovery.Finally,text-mining has been Aselection of Review articles is summarized in Table 1 applied to existing literature databases with the aim to (see Table S1 in the Supporting Information for amore analyze which processes and plastics and in which combina- extensive list of papers) providing an overview of the main tion have received the most research attention up until the processes analyzed in this Review,sorted by the different time of writing (Section 1.4.3). processes for easy reference,along with the key messages from past research. Most notable from this summary is the missing link between research and industrially implemented 1.4. Assessment of the Most-Promising Areas for Future Research processes as well as the prior focus on fuel and monomer 1.4.1. Role of Chemical Recycling in aCircular Economy recovery.Complimentarily,alife-cycle analysis (LCA) is included which assesses the energy requirements and envi- Thefraction of collected household plastic waste is ronmental impacts of selected low technology readiness level between 41 and 76 wt%inFrance,Germany,UK, Spain, (TRL) processes—important parameters for asuccessful and Italy.[11] This plastic can be sorted into main streams of economic and ecological commercialization. Another consid- (PP), high-density (HDPE), low- eration is the mismatch between the purity of researched density polyethylene (LDPE), PET and polystyrene (PS) waste streams and waste streams actually available in current using aseries of rotary screen drums,near-infrared sorting

Ina Vollmer graduated with aMSc.E.P Bert M. Weckhuysen obtained his PhD (2015, US) in Chemical Engineeringfrom degree from K.U.Leuven (Belgium,with Massachusetts Institute of Technology (US). Prof. Robert Schoonheydt) in 1995. After In 2019, she completed her doctorate postdoctoral stays at Lehigh University (PA, research on methane dehydroaromatization USA, with Prof. Israel Wachs) and Texas at Delft University of Technology (The Neth- A&MUniversity (TX, USA, Prof. Jack Luns- erlands, with Prof. Freek Kapteijn) after ford) in 2000 he became full Professor at which she went on to Utrecht University Utrecht University (The Netherlands).His (The Netherlands), where she is currently research focuses on the development and studying the reaction mechanism of chem- use of in situ and operando spectroscopy for ical recycling processes of plastics in the studying solid catalysts under realistic reac- group of Prof. Bert Weckhuysen. tion conditions at different length scales.

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Table 1: Overview of aselection of Reviews covering various methods for waste plastic processing. Title:(published date/ first available online) Process:Key messages: Mechanical and chemical recycling of solid plastic waste[6] Mechanical, -Overview over both mechanicaland chemicalrecycling methods with (November 2017) Pyrolysis comparison of limitations,advantages and disadvantagesofthe different processes -Degradation during mechanicalrecycling limits closed-looprecycling although mitigated through stabilizers and compatibilizers -Design ForRecycling and From Recycling are important in realizing acircular economy for plastic -Cland Ninwaste stream deactivate catalysts in addition to inorganic components blocking pores -Overview and analysis provided of various commercial projects and their status Solvent-based separation and recycling of waste plastics:A Dissolution-Gives details of strong and weak solvents for the various polymer types. review[7] -Solvent extraction from recycled polymer can cause damage to the (June 2018) polymer chain due to thermal stress. -Dissolutionofmixed polymer streamsresults in poorer separation of the target polymer. -Future use of hazardous solvents should be reduced PET by Chemical Recycling:AReview[8] Solvolysis -Polyethylene terephthalate (PET) polymer is difficult to purify once (September 2008) formed, so recycling needs to yield avery pure monomer to allow for repolymerization -Large variety of PET available due to differing degrees of crystallinity -Risks that legislationaims at eliminating polymers that have highest potential for recycling, like PET Chemical recycling of waste plastics for new materials Solvolysis, -Hurdles to commercialization are financial incentives and catalyst production[9] Pyrolysis effectiveness (June 2017) -Unique issues with each type of plastic highlighting the importanceof reducing mixed polymer plastics -Progress in design for recycling of polymers will facilitate chemical recycling Thermochemical routes for the valorizationofwaste poly- Pyrolysis -Reactor design and process conditionscrucial for tuning product olefinic plastics to produce fuels and chemicals. Areview[10] distribution due to heat and mass transfer limitations in processing waste (January 2017) plastic -Importance of (acid) catalyst for reducing reaction temperatures and washing steps.This leads to granulates that can be Tables S2–S8, whilst areport[14] lists additional companies recycled mainly mechanically into non-food-grade plastic specifically regarding polyurethane (PU) recycling. products,such as flowerpots,paint buckets or shampoo .Polyvinylchloride (PVC) and polyamides (PA) are 1.4.1.1. Solvolysis contaminants.This results in recycling rates of 21 to 42 wt% in the aforementioned countries including exports for recy- PET is aspecial case of an already pure mono-stream with cling to other countries.[11,12] Theactual use of recycled plastic as little as 16.4 ppm of PVC,29.4 ppm of other contaminants in new plastic products is only 12.3 wt%inGermany.[13] The (Germany) and 4.1 ppm PA (France) and mechanical recy- collection and recycling schemes of these countries are cling rates can be as high as 15 wt%.[11] Therecycled resin is, forerunners while globally 40 wt%are landfilled and however, mixed with virgin resin to maintain color and 32 wt%leak into the environment.[3] Therefore,there is structural integrity.Alternatively,very well sorted streams aclear need and demand for dramatic improvements in both containing only and polyamides can be depoly- collection and recycling,also because the shipping of waste to merized into monomers by solvolysis (Section 2.2) and used other countries without proper waste management infra- to produce polymer resins by virgin-grade producers. structure for processing raises ecological and ethical concerns. Full depolymerization of PET to the monomer bis(2- How chemical recycling processes can bolster the circu- hydroxyethyl)terephthalate (BHET) by glycolysis is pursued larity of common polymers and avoid landfilling, incineration, by Garbo,[15] IBM,[16] and Dupont-Teijin.[17,18] Thepurified and exports to other countries is illustrated in Figure 1. The monomers can be recycled back to PET granulate. McKinsey report,[2] offers an estimate of the size of waste Ioniqa,[19–21] operating a10kilotons/year facility since 2019, streams in 2030, however dissolution and solvolysis are not employs ionic liquids to assist glycolysis.Rather than mentioned and hence no predictions were included for those depolymerizing PET all the way to monomers,CuRE[22] and processes in Figure 1. Most of these processes are actively PerPETual[23] glycolyze PET chains to low molecular weight developed by companies and start-ups and we gathered (Mw)oligomers.The oligomers can also be repolymerized to information on these companies,conducting interviews with PET granulate.Inthis way,PerPETual recycles 2million several of them (Table S9). Some are highlighted here and in plastic bottles per day in aplant in Nashik, India and CuRE is planning apilot plant in 2020. Instead of BHET,hydrolysis of

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Figure 1. Illustration of an envisionedplastics value-chain that could enhance the transition to circularity.Currently most plastic is incinerated or landfilled (bottom left), because collection and sortingproduce very contaminated and mixed plastic-waste streams. Better techniques for collection and sorting lead to streams of plastic waste that can be recycled by the various chemicalrecycling methods. These routes are still going to be complemented by traditional mechanicalrecycling for the purest steams of asingle polymer.The shares that each of the techniques correspondedtoin2016 bottom left of the waste processing method and prediction for 2030 are shown at the bottom right of the waste processing method. These values are based on the McKinsey report.[2] The plastic objects are sold to the consumer and after its life cycle collected again for sorting to undergo another recycle.

PET yields terephthalic acid (TPA) which is aprecursor for 1.4.1.2. Dissolution/precipitation processes BHET.The Infinia process is an example of this and BP has recently announced plans to build apilot plant based on it.[24] Themost versatile process for dissolution/precipitation Efficient heating for alkaline hydrolysis of PET is achieved (Section 2.3) was developed by Fraunhofer IVV together with with amicrowave-reactor in the Demeto-process operated at CreaCycle GmbH. Their CreaSolv process,[31] which is the pilot scale since 2014 by Gr3n.[25,26] Bio-inspired enzy- licensed to other companies,was tested for expanded- and matic hydrolysis of PET is applied by Carbios.[27] PA-6 can extruded- polystyrene containing hexabromocyclododecane also be depolymerized to yield caprolactam. PA-6 from (HBCD), from waste electric and electronic equipment is hydrolyzed with steam by Aquafil in aplant in (WEEE) plastic[32,33] as well as for packaging.Dissolution/ Slovenia to obtain pure caprolactam recovered by vapor precipitation processes can separate one polymer from condensation, which is then recycled back to carpets.[28] mixtures of polymers as present in multilayer films,often Another solvolysis process,methanolysis is applied by Loop PP/PAorPP/PET[34–36] or WEEE plastics.[37] Forexample,the Industries[29] and in development by Eastman[30] to produce NewCycling process,developed by APK AG,can separate the dimethyl terephthalate (DMT), which can either be used to polymers of multilayer films that also contain aluminum foil produce PET directly or via BHET. by stepwise dissolution in methylcyclohexane of PE and PP while increasing temperature according to the Patent.[38] Since 2018, APK AG runs acommercial plant with acapacities of

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8000 megatons/year and plans to build another one with and (SABIC in partnership with Plastic Energy and Petronas,[47] 25000 megatons/year capacity in 2020. Aspecial category of Fuenix Ecogy and Dow,[48] Shell with Nexus Fuels,[49] BASF separation processes for multilayer films and laminates that with RECENSO GmbH[50] etc.). These resins can be made use solvents,but do not require full dissolution of the into plastic objects by the converter. polymers,were developed by Saperatec in Germany and by Whilst well-researched, the highly mixed product stream PVC Separation in Australia. Saperatec plans to address with hydrocarbons spanning various boiling point ranges from multilayer film separation by reducing the interfacial forces diesel to gasoline and waxes limits applicability of pyrolysis in between PET,PE, and aluminum foil in a18000 tons/year standard reactors,and there is ample room for improvement. plant planned to be operational at the end of 2021. The OMV addresses problems of heat- and mass-transfer encoun- associated Patent indicates use of an organic solvent-based tered in highly viscous and not very heat-conductive plastic, micro-emulsion for swelling and acarboxylic acid to accel- dissolving it in acrude oil fraction before feeding it to their [39,40] 1 [51] erate delamination. TheAustralian company PVC Sep- 100 kghrÀ test pyrolysis reactor. Catalytic Tribochemical aration developed aprocess to delaminate multilayers by Conversion developed by RECENSO GmbH attempts to swelling the polymer in alow boiling solvent. Exposure to hot overcome heat- and mass-transfer limitations via mechanical water, causes the solvent to flash out and release the desired force,which also lowers the reaction temperature required polymers,which can be separated in asifter due to density since mechanical energy is introduced in addition to thermal differences.[41] energy.[50] Theprocess is currently rated at TRL 6/7 and is Dissolution/precipitation processes also allow for the operated at the pilot plant scale.The Cat-HTR process removal of colorants and other additives via filtration to operated for 10 years already at the pilot-plant scale achieves produce higher purity resins,competitive to virgin polymer, avery homogeneous heat distribution, injecting supercritical and to recover valuable additives.PolyStyreneLoop recycles water into the plastic waste.[52] Thesupercritical water also polystyrene foam demolition waste by removing HBCD not quenches unwanted side reactions leading to high yields of only recovering polystyrene but also elemental bromine, stable hydrocarbon liquids.This process is insensitive to which can be used for the synthesis of new BFRs.The moisture and contaminations of the plastic with other organic construction of aplant with an annual capacity of 3300 tons waste.Acatalytic process is applied by the Dutch/Indian input has started in December 2019 in Terneuzen, the company Patpert Teknow Systems with 40 installations vary- Netherlands.[32,42] ing in process capacity between 110 and 7300 tons/year.[53] Thechoice of an environmentally benign solvent with Theplastic is co-fed with asilica/alumina based cracking high dissolution capacity that can be easily recovered often catalysts to the pyrolysis reactor at 350–3608C.[54] Heavy wax decides the fate of the development of adissolution/precip- fractions are separated from other products and fed to itation process.For example,PVC waste recycling with asecondary catalytic cracking unit, followed by an integrated butanol as solvent and steam as anti-solvent by VinyLoop fractionation column containing acatalyst fixed bed. was shut down after more than 15 years of operation, because the process was not effective enough to remove additives, 1.4.1.5. The future of chemical recycling such as plasticizers.[43] However,some innovative solutions are developed. Agreen high-boiling aromatic solvent, Based on the brief overview and the compiled list of cymene,derived from citrus fruit industry waste streams is companies (Table S2–S8) active in the field of chemical used for recovery of PS from packaging by technology recycling, it is clear that more selective solvolysis and licenser Polystyvert.[44] Theprocess of PureCycle with pyrolysis processes are required and that catalytic processes afacility under construction dissolves PP in supercritical have not found much application to date.More innovation in butane followed by precipitation upon decompression.[45] the field of upcycling is also required to create more business opportunities.Itisinteresting to note that pilot plants for 1.4.1.3. Upcycling solvolysis,[55] pyrolysis,[56] and hydrocracking[57,58] were plan- ned or already existed in the nineties,however when Producing chemicals from plastic waste that are more contacting them, alack of suitable waste streams,logistics valuable than monomers,polymers,orthe feedstock of steam- and commercial viability were cited as reasons for discontin- crackers is an alternative to valorize plastic waste (Sec- uing the project or knowledge of these processes was not tion 2.4). BioCellection makes organic acids from PE that are available anymore.Specifically,the price competition of essential in the production of performance materials,such as pyrolysis oil with crude derived petroleum hinders commer- solvents and coatings.[46] cial viability.Due to recently renewed interest, anumber of new market entrants are at asimilar stage of development as 1.4.1.4. Thermal routes in the 1990s.Processes are usually developed in demonstra- tion plants and then licensed out. As crucial in determining Pyrolysis (Section 2.5) takes relatively mixed plastic waste the fate of this new wave,contacted companies name the streams,but handle only small amounts of other organic, implementation of appropriate policy frameworks,such as

PVC,PU, and PET impurities depending on the process.It atax on CO2 with clear definitions of chemical plastic typically yields fuel-like products.Inpartnership with poly- recycling (Tables S2–S8). Abetter integration and collabo- mer manufacturers,the pyrolysis oil is further upgraded to ration of all stakeholders along the value-chain as well as produce the monomers for making virgin grade resins amore forward-looking investment strategy,standardization,

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and legislation are necessary.CO2 emission penalization/ GF.Emissions from transport of waste are minimal. Energy

taxation or other measures that integrate environmental recovery avoids 30–45 wt%CO2 emissions,which would arise costs,are needed to drive market interest and further from traditional electricity generation. adoption of chemical recycling. Likewise,more stringent All alternative treatments to incineration reduce process

packaging design regulations that dictate better recyclability EoL CO2 emissions,since production of anew resource is

and stricter regulations on waste disposal would pave the way avoided. CO2 emissions of different EoL treatments indexed towards acircular economy. against incineration without energy recovery are ranked by

the potential to save CO2 emissions (Figure 2). 1.4.2. Life-Cycle Analysis of Chemical Recycling Processes and In case plastic waste is landfilled, embodied carbon is Plastics hardly released over time (< 2wt.%)[60] which reduces life-

cycle CO2 emissions to athird compared to incineration Circular end-of-life (EoL) treatments aim at reusing the without energy recovery.But it has to be noted that landfilling plastic directly or as aresource,for example,through is not circular, requires alot of land and causes toxic chemical or mechanical recycling, whereas plastics are substances to leach into the soil and groundwater unless merely disposed in linear EoL treatments,such as inciner- prevented by costly measures.

ation or landfilling. We analyzed the environmental benefit of Fordissolution/precipitation, CO2 savings are the highest

these EoL treatments in terms of CO2 emissions,which arise at 65–75 wt%, because no bonds are broken or need to be

from resource and energy needs and relate to climate change reformed. With solvolysis similar amounts of CO2 can be as well as depletion of fossil resources and reduction in air saved due to an efficient conversion into ahigh-value quality.Adiscussion of other indicators such as toxicity, recyclate,possible,for example,for uncontaminated PET eutrophication, and water-demand requires amore detailed waste.Pyrolysis products can replace fuel oil and natural gas [59] study beyond the scope of this Review. LCA is performed which avoid 30 wt%ofthe CO2 emitted during incineration. in reference to one ton of plastic waste having experienced Mechanical recyclingofmixed plastics with additives only

the full life cycle of plastic production, product manufactur- avoids around 25 wt%ofCO2 emissions due to the low ing, , transport, sorting,and recycling also recyclate quality.

accounting for losses in each of those steps.Webenchmark CO2 savings only come from avoided plastic,pyrolysis oil, the recycling options and EoL treatments listed below against or energy production. Recycled products still need to be waste incineration and landfilling: remanufactured and transported, meaning that reuse is 1) incineration with energy recovery advantageous over all recycling routes.However,electrifica- 2) pyrolysis (see also Section 2.5) tion of these processes via renewable energy supply could be 3) mechanical recycling capable of de-fossilizing process schemes and steps but is not 4) solvolysis (see also Section 2.2) directly related to recycling and thus not included in the 5) dissolution/precipitation (see also Section 2.3) analysis.Itisalso worth noting that future polymer produc- tion may look to sustainable monomer sourcing pathways,

Chemical recycling could complement mechanical recy- that is,via CO2-derived methanol, thus reducing emissions cling,addressing mixed polymer and composite plastics that profiles still further for certain products. can only be recycled to lower quality products.Therefore,we While the analysis identifies benefits of certain EoL analyzed EoL options for multi-material plastic waste with technologies over others,itneeds to be extended to more diverse applications and meaningful volumes: plastic waste streams and chemical-recycling technologies, 6) acrylonitrile butadiene styrene (ABS) from aback panel including emerging technologies (Section 2.1). However,the often used in electronics results indicate,perhaps logically,that the less the polymer 7) high-impact polystyrene (HIPS) from acase used for structure/bonds are broken and the higher the quality of the food products recycled product, the better the environmental performance.

8) PET from drinking bottles Moreover,internalizing CO2 costs in the product price could 9) PET-PE multilayer food packaging foil make circular options more profitable than polymer produc- 10) glass fiber reinforced polypropylene (PP-GF) from tion from fossil sources.Dissolution/precipitation for example

acomposite door panel used in transport. avoids between 3to6tons CO2 for each ton of plastic waste.

If this CO2 were to be priced at 50 to 200 Euro/ton, recycled

With incineration, 5to10tons of CO2 are released by plastic product would have aprice advantage of 150 to 120 1ton of plastic over its life cycle. Variations are mainly caused Euro per ton of plastic waste.This is significant, considering by differences in carbon content and energy requirements of that the typical polymer value is around 1500 Euro/ton.[61] production or the different plastic types (Tables S10–S11), because melting point, viscosity,tensile strength, and energy 1.4.3. Analysis of the Most-Researched Processes and Plastics content affect process parameters.Approximately half of life-

cycle CO2 emissions stem from the plastic production, while We analyzed 474 relevant research articles by using

less than athird of CO2 emissions arise from embodied carbon acustom text-mining script using the Scopus application released by incineration. Theremainder is related to final programming interface (API) to understand which chemical product assembly,amounting to less than 10 wt%for multi- recycling processes have been used for which type of plastic layer packaging and around 25 wt%for the composite PP- and how much research attention they have received (Fig-

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Figure 2. Top: CO2-equivalent emissions of different EoL treatment technologiesapplied for several plastic-waste streams, in relative emissions indexed to incineration (100 wt%). Bottom:CO2-equivalent emissions of different EoL treatment technologiesinabsolute emissions in ton CO2/ ton waste by life -cycle stage. ure S1). Surprisingly,the most researched plastics are not the Pyrolysis is by far the most mentioned process and often plastics produced in the highest volume.PET is by far the appears together with cracking.Itcan, however, be seen that most researched plastic type,appearing more than 2000 times catalytic pyrolysis and cracking have received far less in the searched literature.This is presumably because attention, although catalytic pathways can greatly influence collection systems are already in place for PET,raising the recyclate product scope (Section 2.5). Another interesting awareness in the public. Theavailability of plastic waste process that remains under-researched is hydrocracking (mono-)streams seemingly dictates research effort. In con- (Section 2.5.3). Finally,very little research has focused on trast, HDPE having twice the share of plastics production[14] real waste streams,such as plastic solid waste (PSW) and compared to PET only appeared 353 times.Itisgenerally municipal plastic waste (MPW). more energy intensive to break the strong C Cbonds of À polyolefins selectively compared to the ester bonds in PET, which can be depolymerized at relatively mild conditions with high selectivity to monomers.

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Figure 3. Microwave heating (b), plasma reactors (c) and supercritical fluids (d) can address some of the problems encounteredinconventional solvolysis and pyrolysis (a).

2. Problems and Novel Solutions can also depolymerize polyolefins,which have achemically very inert backbone that is harder to cleave than ester and 2.1. Emerging Technologies acid amide bonds present in PET and PA respectively.By tuning the applied temperature and pressure of the super- Several unconventional approaches to polymer recycling critical fluids,such as,alcohol and water, solvation (as well as are discussed in the literature.Some use alternative ways to basic/acid properties for water) can be altered, making them supply energy for the depolymerization as in mechanochem- catalytically active.[64,65] This was reported for treatment of ical approaches,photo-reforming as well as microwave and composites such as fiber reinforced plastic.[66]

plasma reactors.Bio-inspired routes include polymer diges- Under supercritical conditions (i.e.H2O) even polyolefins tion by enzymes.There are also efforts to use alternative can be depolymerized. TheCCbonds in the polyolefin À solvents,such as supercritical fluids,orionic liquids (ILs) and backbone are harder to break that ester or acid amide bonds deep eutectic solvents.These approaches attempt to address in PET and PA.This is often called pyrolysis under super- problems encountered in conventional pyrolysis and solvol- critical conditions.However,incomparison to pyrolysis ysis (Figure 3a), namely low heat conductivity of plastic and (typically performed under an inert atmosphere), supercrit- general heat distribution problems in industrial reactors;and ical water or alcohol are not inert and thus hydrogenated or the issue of arelatively small contact area between the solid oxygenated products are formed during treatment.[66] An plastic and the reactive media or catalyst. Themethods advantage of supercritical fluid treatment of polyolefins is the described in Figure 3b–d have the potential to achieve more relative insensitivity to organic impurities and moisture efficient heating, higher conversion and higher selectivity. usually present in PSW as demonstrated by the Cat-HTR Conversion under microwave irradiation (Figure 3b)and process.[52,67] Theuse of plasma-assisted pyrolysis,which often in supercritical fluids (Figure 3c)often lead to more homo- leads to higher monomer yields is discussed in further detail in genous temperature profiles throughout the reactor. This can Section 2.5. lead to faster heating rates and amore controlled and ILs,aswith supercritical fluids,address the problem of the selective reaction, due to similar reaction rates at all locations small contact area between the plastic and the reactive media of the reactor. Reactions are also sped-up in several reported and/or catalysts in conventional solvolysis and pyrolysis.Use cases.For example,microwave reactors can lead to shortened of ILs for solvolysis is discussed in Section 2.2. ILs are already reaction times during solvolysis,but the use of acatalyst is applied commercially for other processes.For example,the required to achieve high selectivity to monomers.[62,63] This BASIL process by BASF to scavenge acids in the synthesis of approach is discussed in more detail in Section 2.2. This rather alkoxyphenylphosphines was commercialized in 2002 and has novel way of reactor heating was also applied to pyrolysis, shown to be much more environmentally friendly than the discussed in Section 2.5. previous process using tertiary amines.[68,69] It was found that Solvolysis in supercritical fluids is also compared to with ILs,PEcan be cracked exclusively to alkanes in yields of conventional solvolysis approaches in Section 2.2. In contrast up to 95%atbelow 2508Calthough relatively slowly.[70]

to normal solvolysis conditions,supercritical fluids (e.g. H2O) While ILs are often used to replace organic solvents (they

15410 www.angewandte.org  2020 The Authors. Published by Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim Angew.Chem. Int. Ed. 2020, 59,15402 –15423 Angewandte Reviews Chemie have negligible vapor pressure) and can act themselves as of plastic waste to valuable products,which was previously catalysts (Section 2.2 and 2.3) they suffer from high toxicity, performed with aprecious-metal or Cd-based photocata- poor biodegradability and often high costs.[71] Aproposed lysts.[93] alternative are eutectic solvents.These are two solids,which when combined have alow melting point and can act as acid 2.1.3. Biotechnology or base through functionality choice.[72] This class of solvents has been used for PET glycolysis leading to high selectivity to Naturesown catalysts,enzymes and enzymatic collectives the monomer BHET.[73] in cells and microorganisms,have been shown to able to degrade plastic waste.[94,95] Enzymatic hydrolysis of polyesters, 2.1.1. Mechanochemistry that is PET,can be used both to functionalize the surface of the plastic and to fully depolymerize it.[96] Degradation of Drawing inspiration from the field of polymer manufac- several plastics by microbial enzymes was reviewed by Wei ture,the application of mechanical stress has been shown to et al.[97] and the recently published Plastics Microbial Bio- lead to the homolytic cleavage of polymers and thus radical degradation Database (PMBD) features alist of 949 micro- formation. This can lead to cross-linking and cross-polymer- organisms–plastics relationships referring to 79 specific genes ization, and can restore the properties of plastic,but it can that are known to be related plastic biodegradation.[98] also be used to promote depolymerization.[74–86] No additional Biodegradation pathways relate to chemical “familiarity” heating is required, because the necessary heat is generated at and enzymes act on chemical bonds that are found naturally, the ball impact zone. that is,glycosidic,amide,and ester bonds.With respect to Mechanical stress can occur in shear-reactors,ball mills,as “Design for Recycling”concepts this is an important consid- well as during sonication (e.g.imploding bubbles). The eration and makes addition polymers,such as,polyolefins depolymerization of poly(phthalaldehyde) was achieved by more challenging. imploding cavitation bubbles formed during sonication. A Mw While aliphatic polyesters are known to be enzymatically dependence was observed. When starting with high average hydrolyzed even at room temperature,anenzyme for the 1 Mw polymers (458 kgmolÀ )depolymerization was faster than aromatic PETwith appreciable rates was discovered 1 [82] [99] for lower Mw fractions (< 26 kgmolÀ ). Sonication was in 2005 by Müller et al. Many researchers investigated the performed under Ar at alow temperature ( 408C; t = 6h)to enzymatic hydrolysis of PET using cutinases and its muta- À counteract heat generation, leading to 60%depolymerization tions.The full degradation of PET plastic waste and recovery in the high Mw case.This highlights alimitation of mecha- of monomers was reported using the bacterium, Ideonella nochemical bond scission. There is alimiting Mw,under which sakaiensis 201-F6 discovered near aPET recycling the chemical energy generated by the applied mechanical plant and its expressed hydrolases,PETase and MHETase (a force is not sufficiently accumulated to break covalent mono-(2-hydroxyethyl)terephthalate-digesting enzyme).[100] bonds.[87] Themutation of two active-site residues of cutinase are Mechanochemical effects can also be used to pre-treat responsible for PET degradation back into the monomers: waste plastics,facilitating pyrolysis.For example,the need to BHET,and TPA.[101] MHETase is responsible for the further remove halogen impurities which cause generation of corro- hydrolysis of MHET to TPAand ethylene glycol (EG), which sive and toxic gasses during pyrolysis can be avoided in this can be used in PET production. Generally,crystalline PET way.The energy demanding de-chlorination of PVC and cannot be hydrolyzed by enzymes and enzymatic hydrolysis is removal of bromine additives from PS has been achieved thus preferentially performed above glass transition temper- using ball milling.[88] Ball-milling assisted de-chlorination of ature,between 67 and 818Cdepending on the crystallinity. PVC can be performed in the presence of dry CaO,which Enzymatic depolymerization of PET and the biodegradable itself becomes chlorinated and can subsequently be removed polymer PLA has been commercialized by Carbios.The by washing.[89] Theprocess can also be run in wet and basic Patents describe an pre-amorphization step[102] and the use of conditions.[90] As another pre-treatment for pyrolysis,asteam- several enzymes.[103,104] Polyurethane varnish and polyether explosion treatment has been shown to lower the liquefaction polyurethane foams were degraded by up to 87%after temperature by 508Cfor PS and by 1008Cfor HDPE.[91] 14 days of incubation through amicrobial enzyme of afungi, C. pseudocladosporioides strain T1.PL.1.[105] In addition, 2.1.2. Ambient-Temperature Photo-Reforming enzymes were recently also found to be useful for synthesis of biodegradable plastics.[106] While not many commercial Regarding the utilization of (direct) photonic energy, processes based on enzymes exist to date,further discoveries Reisner et al. reported on ambient-temperature photo- and advanced protein-engineering techniques through which reforming of plastic waste into fuel and bulk chemicals.Cut further mutants may be accessed might make enzymatic pieces of raw polymer or fibers of PET and polylactic acid depolymerization aviable technology in the future given the (PLA) with and without food contamination were converted low temperature required and thus small energy require- under aqueous alkaline conditions,using an inexpensive,non- ments. [92] toxic photocatalyst. Ni2Psupported on cyanamide-func- tionalized C3N4 was reported to promote efficient charge separation with aphotostability of ca. 120 h. This approach represents an elegant, simple and low-energy transformation

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2.1.4. Design for Recycling PET can be depolymerized to the monomers TPA, BHET, or DMT,depending on the process (Figure 4). While BHET Polyolefin-based waste has historically been treated via and DMT can be directly used for the polymerization to PET, gasification or unselective pyrolysis.There is renewed interest TPAneeds to be converted into BHET first. Nevertheless, in the conversion of PE and PP back into monomer and/or this process has replaced the polymerization from DMT more oligomers.This could be more easily achieved, if the plastics and more.The BHET that is obtained from glycolysis is not are designed with recycling in mind, for example,byavoiding pure enough to produce PET directly and oligomers are additives that lead to degradation during melting and re- produced as well that can be used to synthesize PU or extrusion or that cause catalyst deactivation in thermal unsaturated polyester resins (UPR).[124] Thus,hydrolysis processes.Mülhaupt et al. discuss this so-called “design for which allows the production of high purity TPAappears recycling” approach[107] for polyolefins.[108] This Review and very attractive.However,high purity BHET can be obtained other literature from Mülhaupt et al.[109] discuss the produc- employing ametal acetate[125] or sodium carbonate[126] catalyst tion of 100%polyolefinic plastics and composites without in glycolysis,the sodium carbonate produced almost an 80% additives.This can be achieved with multisite polymerization yield after 1hat 1968Cwith areported EG:PET molar ratio catalysts and specialized injection-molding processes,such as of 7.6:1. oscillating packing injection molding.This approach removes In contrast to PET,PUsolvolysis often yields other the necessity of producing different polymers in different chemicals that cannot be used directly to make the same PU plants,avoids the use of additives,such as glass fiber, and again, but another type of PU.[117, 118] It was discovered that generates aproduct that could potentially be depolymerized using both water and glycol as cleavage agents,so-called with polymer synthesis catalysts like Ziegler–Natta. The hydroglycolysis,yielded amore pure polyol.[127] Afurther synthesis and production of PP or PE-based plastics or demonstration of hydroglycolysis was to obtain TPAfrom composites in this manner,[110] can potentially be more PET with reduced energy input.[128] Forthe hydroglycolysis of impactful with regard to production efficiencyand ecological PU,however,the higher processing cost associated with footprint. separation was noted. An alternative and inspiring approach for the production Microwave heating has been developed for more efficient of polymers with tunable degradation properties was recently heat delivery.This technique can provide amore even discovered by Shieh et al.[111] In this study,silyl ether-based temperature profile inside the reactor, as it heats volumetri- cyclic olefins were copolymerized with norbornene deriva- cally.This leads to faster depolymerization, an example with tives to produce copolymers with varying stability when 1-pentanol as the solvent utilized microwave heating to exposed to HCl. Whilst not yet widely applied, the ability to reduce the time for complete decomposition of PET using design polymers that degrade under pre-determined condi- aKOH catalyst from 32 min under normal heating to just tions into their monomers (or oligomers) is akey step forward 2.5 min.[128] Microwave assisted glycolysis of PU[129] and in the recycling of plastics. hydrolysis of PA [130] has also been reported. However,the purity of the product might not be the same as under normal heating and the differences in products are rarely reported in 2.2. Solvolysis much detail. Microwave assisted non-catalytic glycolysis of PET lead to amixture of oligo-esters with unknown purity.[62] As mentioned in Section 1.4.1, solvolysis is apotentially While in the non-catalytic microwave hydrolysis of PET, more selective way to recover monomers from polyesters and 100%yield to TPAand EG was achieved over a120 min polyamides employing lower temperatures compared to those reaction.[63] To understand, whether microwave-assisted sol- used in pyrolysis.The solvolysis processes hydrolysis,alcohol- volysis is more economical than conventional heating, ysis (glycolysis and methanolysis), phosphorolysis,ammonol- adetailed comparison of power usage would be beneficial ysis,and aminolysis cleave ether, ester,and acid amide bonds as this was found to constitute the highest operating cost.[131] and therefore are limited to polymers with these bonds.Much In the case of biodiesel production from palm oil, the energy R&D focusses on PET[112–116] and PU[117, 118] and to alesser consumption was found to be halved when employing extent on PA,polycarbonate (PC)[119] and PLA[120] depolyme- amicrowave reactor.[132] However, since microwave heating rization. Theadvantage of these processes lies in the is relatively inefficient, the overall power requirements stay possibility to obtain monomers that can be further purified, similar.[133] Scaling up was found to have apositive effect on filtering out additives and colorants,allowing for re-polymer- energy efficiency.[134] Thefact that companies like Gr3n[26,25] ization to virgin-grade quality.This is especially interesting and Pyrowave[135] are currently developing this technology at for PU,which cannot be recycled mechanically.Ifthe purity commercial scale shows the promise of this technology. or quality of the recovered monomers is not as good as the Other issues encountered in all solvolysis processes are monomers initially used, they can also be mixed with conven- 1) separation of the liquid cleavage agent and other by- tionally obtained monomers for the polymer synthesis.Afew products, commercial plants exist for methanolysis of PET[121] and 2) small contact area between the liquid cleavage agent and glycolysis of PET and PU.[118] Thefurther development of the solid polymer,and these processes over hydrolysis were ascribed to higher 3) recovery of dissolved catalysts. energy requirements of hydrolysis,because of high operating temperatures needed.[122, 123]

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Figure 4. Products obtained through the different solvolysis pathways of PET,PU, and PA and how these products can be used to recycle back to the polymer or to obtain valuable products.

Re:1)Aminolysis,phosphorolysis of PU and if in excess Re:3)Catalyst recovery can be addressed using solid- of glycol also glycolysis,lead to phase separation of the acid catalysts[142] and manganese oxide based catalysts for products from the solvent. Alternatively,the monomers can glycolysis of PET.[143–146] Theyield of BHET using S/Zn-Tiat be recovered by crystallization, distillation or by liquid 1808Cfor 3h was found to be 72%, which is close to that extraction, that is,with water. Owing to the high cost of achieved using homogeneous catalysts.[142] While comparison using excess glycol, commercial plants only employ single- to non-catalytic glycolysis was not always provided, some phase glycolysis of PU,which leads to less pure polyols.To demonstrated an increase of reaction rate and higher BHET improve process economics,deLucas et al. employed crude monomer recovery,especially for high surface area catalysts, glycol, awaste by-product from the diesel industry,which lead such as mesoporous metal oxide spinel catalysts.[143, 144] to an even purer product than the best performing glycol Another novel approach employs ILs,[147] which can act as EG.[136] strong solvation agents for polymers.[148,149] Although applied Re:2)The small contact area between liquid and solid commercially in afew cases ILs can have ahigh toxicity and plastic pieces retards the reaction, which is why the reaction LCA has shown that they only become viable,when separated proceeds much faster at temperatures exceeding the plastic more efficiently from the reaction product.[150] When melting point.[115] An approach to lower the reaction temper- employed with the right counter-ion, depolymerization prod- ature is reactive extrusion, which also decreases reaction ucts can be separated by liquid–liquid extraction.[151,152] times from hours to minutes.[124,137] Theglycolysis of PET via Distillation however is limited, because most ILs used for reactive extrusion led to low molecular weight oligo- depolymerization are not volatile.Non-neutral ILs,that is, esters,[124,138] which could be used to make PU.However, basic 1-butyl-3-methylimidazolium acetate [Bmim][Ac] can more research effort is needed to make this process more act as catalyst and fully glycolyze PET.[153] However BHET selective to BHET.Another approach uses aphase-transfer monomer yields typically reported with ILs do not exceed catalysts that was shown to transport sodium anions from 60%. Sub- and supercritical and depolymerization using ILs NaOH to the plastic surface of different nylons and PET in is also addressed in Section 2.1. basic hydrolysis.[124] ForPET,the yield increased from 2to 90%over the reaction time of 5hat 808C. Organocatalysts, such as the quaternary ammonium salts,are used as phase- 2.3. Dissolution/Precipitation transfer catalysts[139] and can assist glycolysis of PET via hydrogen bonding.[140] In addition, performing hydrolysis or Dissolution/precipitation offers the possibility to recover methanolysis in supercritical fluids addresses the problem of polymers from plastic waste that are free of additives,such as plastic/cleavage-agent contact and provides homogenous and pigments.Other additives,such as flame retardants,can even fast heating.[66,141] be recovered for reuse.[154] This can be achieved using asingle solvent or acombination of asolvent and an anti-solvent

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Figure 5. The plastic is dissolved and undissolvedfragments, such as pigments, are removed by filtration. The top route describes the dissolution/precipitation technique using asingle solvent, which is removed by evaporation, crystallizing the polymer for recovery.For the bottom route an anti-solventisused to precipitate the polymer,which can be recovered by filtration. Both routes require asolvent removal step, which can be time and energy consuming unless asupercritical anti-solventisemployed.

(Figure 5).[7] Forthe solvent/anti-solvent approach, the sol- removed by mild heating to obtain the solvent in anon- vent selectively dissolves aspecific polymer,ananti-solvent is polar form for reuse. then added to precipitate out the polymer for recovery.In Fororganic solvents,dissolution can be relatively slow between dissolution and precipitation steps,non-dissolved owing to the small plastic/solvent contact area. Similarly,to materials,such as pigments,are separated from the polymer solvolysis,the process could be accelerated by microwave solution.[155] Themixture of solvent and anti-solvent obtained heating (Figure 3b)orultrasonic irradiation drawing inspira- must be separated again for re-use in the process.Such tion from related fields.Ultrasound assisted cellulose dis- aseparation is energy and time consuming for solvents with solution was achieved in ILs[165] and starch dissolution in ahigh boiling point. DMSO.[166] Microwave-assisted digestion has been described Another problem is the complete removal of solvents,as for analytical purposes of samples of PE, PP,PVC,and ABS any residual solvent affects polymer properties.[156] Solvent in amixture of hydrogen peroxide and nitric acid.[167] In removal and separation is easier if one of the solvents is addition, breaking up the polymers into oligomers may also asupercritical fluid as they readily evaporate when the improve solubility in different solvents.This can be achieved pressure is decreased. An example of asolvent/anti-solvent through mechanochemical treatment, for example with system employs formic acid as asolvent and either super- exploding ultrasonic bubbles (Section 2.1.1).[82] [157] [158] critical dimethylether or CO2 as anti-solvent. Afurther This approach in particular, but also other dissolution/ example achieved 97%extraction of BFR from atypical precipitation techniques,yields acrystallized polymer that WEEE plastic, HIPS dissolved in d-limonene,which is an needs to be upgraded to obtain reliable virgin grade quality. environmentally friendly solvent.[154] Theextraction was Over its lifetime the polymer degrades through various carried out at 658Cand 20 MPa, with a2:1 ratio supercritical mechanisms,such as photo-oxidation and mechanical stress,

CO2 to d-limonene.Very few polymers fully dissolve in leading to areduction in chain length. Themolecular [159] supercritical CO2 and therefore it can be used to remove structure,phase morphology,and mechanical properties of impurities,such as volatiles,flame retardants,stabilizers and plastic waste streams can be improved by re-stabilization, dyes from polyolefins,polystyrene,ABS and PVC,recovered rebuilding of the macromolecular architecture,compatibili- from packaging,WEEE and flooring waste.[160–162] Using zation of mixed recycled blends and addition of elastomers [168] supercritical CO2 has the added benefit of avoiding toxic and fillers. In addition, the precipitated polymer has to be organic solvents,such as xylene,acetone,toluene,and n- extruded to produce granulate to manufacture new plastic heptane.[163] items. An elegant process for dissolution and precipitation has More research is needed to obtain higher and more been reported employing aswitchable hydrophilicity solvent reliable recyclate quality.Itcould also be envisioned that the (N,N-dimethylcyclohexylamine) that dissolves LDPE in its solvent/anti-solvent system is expanded to separate plastic non-polar form at elevated temperate.Upon cooling of the mixtures using several solvents that dissolve only certain type

solution and with the addition of CO2 and water, the solvent of polymers.Inaddition, further optimization of solvents and becomes polar and the LDPE precipitates.[164] After filtration solvent/anti-solvent combinations is critical. Thesolvent

to remove the precipitated polymer,CO2 and water is should:

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1) selectively and strongly dissolve the desired polymer and 2.5. Thermochemical Routes and Pyrolysis no other added polymers or additives; 2) not be hazardous;and Pyrolysis as applied to plastics is the thermal decompo- 3) easily evaporate for recovery and recycle. sition in an oxygen-free environment into amixture of products similar to the ones obtained by fractional distillation of crude oil, which range from refinery gasses,through 2.4. New Avenues Towards Value Added Chemicals via Upcycling gasoline/naphtha and diesel to immobile residues.Sharma of Waste Plastic et al. pyrolyzed HDPE plastic bags at 4008Ctoproduce adiesel yield of up to 47 wt%, with only the minimum One approach to overcome the often inhibitory low cost oxidative stability and density failing ASTM D975 and EN of virgin monomer is to convert waste plastics/polymers into 590 standards,[188] making it highly suitable in blending for useful, value-added chemicals and/or materials (so-called direct fuel. In contrast, monomer recovery rates for HDPE “Upcycling”).[169] Whilst reports have described the use of are typically below 40 wt%.[189] waste plastic/polymers as the basis of,for example,trans- Pyrolytic products can be used as fuel or converted into parent conducting films for photovoltaics,[170] battery electro- monomers through the processes similar to the established des,[171,172] and carbon nanotubes,[173–175] this Section will focus routes of producing monomers from crude oil, such as steam on the synthesis of chemicals and platform molecules. cracking.Itwould, however,bedesired to produce monomers Instead of depolymerization to monomers,plastics can be from pyrolysis directly.While this is almost impossible for broken down into specific oligomers,which can be used, for PET and PVC,high monomer recovery can be achieved for example,asadditives to tune the properties of inks and polymethylmethacrylate (PMMA) and PS (Table 2). For coatings for the printing industry making them glossier,easier PVC,the activation energy for the evolution of HCl is less to print out and more durable,aprocess patented and used by than half of the energy required to subsequently break the the company GreenMantra.[176] Other reported examples of polyene chains.Therefore,itiskinetically challenging to upcycling of polymers into more valuable materials include selectively cleave the C Cbonds required to recover the [190] À the synthesis of fiber-reinforced plastics,via combination of monomer. Thetemperature of maximum mass loss (TMAX) depolymerized PET with renewably sourced, bio-derived for HCl formation and C Cbond cleavage can be found in [177] À olefinic acids. Kamimura et al. described the treatment of Table 2. Thesignificant difference of the TMAX at which the

Nylon-12 and Nylon-6 with supercritical CH3OH to produce two phenomena occur can be used to pre-treat MPW methyl w-hydroxydodecanoate,afatty acid ester derivative containing PVC,selectively removing HCl.[191] Interestingly, with potential antimicrobial agent applications with 85% this shows similarities to biomass pyrolysis in which the yield.[178–181] With regard to polyolefins,Hakkarainen et al. variable thermal stability of hemicellulose,cellulose,and reported the selective conversion of HDPE to afew lignin is exploited to selectively depolymerize the respective well-defined products,namely,succinic,glutaric, and adipic fractions.[192] acid through microwave assisted acidic hydrolysis.[182] The Forpolyolefins there is some scope for increasing acids were then converted into plasticizers to be used in PLA monomer recovery.Some examples of promising approaches processing.[182] Succinic acid is highlighted by the US Depart- for monomer recovery from PP are two step catalytic ment of Energy as akey platform for the bio-economy pyrolysis which has achieved 35 wt%monomer yield,[193] market,[183] while adipic acid is currently the most common whilst an induction-coupled plasma reactor was capable of dicarboxylic acid produced from petroleum refining[183, 184] generating a75wt%monomer yield[194] (this value was stated with the global market size estimated to be higher than in vol%and converted into wt%assuming ideal gas law). 2.7 megatons per year at aprice of above 1500 Euro/ton. The Research in apilot scale facility (200–500 kg per run) found company BioCellection saw this business opportunity and similar yields of ethylene (36 wt%) and propylene (18 wt%) developed aprocess with adual-catalyst system.[46] Plasti- from mixed polyolefin waste,suggesting that adirect plastic cizers can also be obtained from recycling PET using eutectic to monomer plant could be feasible.[195] Commercial plastic solvents as catalysts.[185] 5wt% of acholine chloride-based pyrolysis plants (i.e.Plastic Energy,Petronas,Fuenix Ecogy, eutectic solvent (ChCl/Zn(Ac)2)was used together with 2- and Nexus Fuel) do not currently aim for the production of ethyl-1-hexanol as solvent to obtain 100%conversion of PET monomers,but for fuel. It should be noted that values can be with ayield of 84.7%ofthe plasticizer dioctyl terephthalate. difficult to compare among different references because of Another approach for the production of higher value inconsistencies in reactor designs and methods of calculating products from post-consumer plastic is the post-polymeri- yield. zation modification to yield polymers with altered adhesion Yields of recovered monomer depend highly on reactor or surface tension properties.[186] Forexample,fluoroalkyla- design (Section 2.5.4), because this influences temperature tion of polystyrene foam waste with photocatalytically profiles and residence time.These two key parameters generated electrophilic fluoroalkyl radicals was successfully determine the product scope for pyrolysis,illustrated for achieved to induce ahigher water repellence.[187] HDPE and PS in Figure 6. Additionally,pyrolysis is typically performed at atmospheric pressure because of the increased coke and heavy fraction formation at elevated pressures.[212]

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Table 2: Plastic type with indicative monomer recovery rates from single stage pyrolysis rounded to the nearest 5%.[a] Plastic type:Maximum pyrolytic monomer Temperatureof: [8C] Product(s) with greatest yield: recovery rate:[wt%]

T5% TMAX

[196] [197] [198] [196] PET ca. 0 400 435 Benzoic acid, CO, CO2 and solid organics HDPE 40 (7808C, 1.3 s, –)[189] 415[199] [b] 460[199] pyrolytic crude oil[200] PVC ca. 0[201] 270[202] 290[202] ,465[202] [c] HCl, Benzene[201] LDPE 40 (8608C, 0.6 s, –)[203] 375[204] 460[204] pyrolytic crude oil PP 30 (6508C, –, –)[205] 355[206] 470[206] pyrolytic crude oil 45 (6508C, –, –)[205][d] in Ar in Ar PS 70 (5008C, short, –)[207] 400[209] [b] 435[209] Styrene[207] 85 (5008C, short, –, vacuum)[208] PMMA >95 (4508C, short, –)[210] 260[211] [b] 360[211] Methyl methacrylate[210]

[a] Temperatures at which 5wt.%mass loss occurs (T5%)and at which mass loss rate is the highest (TMAX)obtained from TGA results rounded to 1 nearest 58Cand measured at 108CminÀ in N2 unless stated otherwise. [b] denotesthat value was interpolatedfrom agraph. [c] denotes the second maximum mass loss temperature for PVC as there are two separate degradation regimes. [d] The reported total yield did not account for carbon deposits left in the reactor.

ing explain the increased yields of monomers at higher 2.5.1. Catalytic Pyrolysis temperatures.[114,216] Catalysts have the potential to both reduce pyrolysis 4) Longer residence times enhance secondary reactions,such [213] as hydrogen and carbon transfer within carbocations, temperatures and narrow the product distribution. How- [212] ever,high viscosity,low thermal conductivity,and relatively leading to cyclisation and aromatics formation. long molecular chains lead to asmall catalyst/polymer contact 5) Stronger acid sites lead to aswitch from random scission to area, as well as inhibit heat and mass transfer.[10] In some cases chain-end scission as the initiation step,which is preferred these issues can be overcome with novel catalyst design;for when aiming at shorter chain hydrocarbons.High mono- mer yields can be obtained from PMMA pyrolysis because example,Ptnanoparticles were deposited on SrTiO3 nano- of the dominance of chain-end scission reactions as cuboids through atomic layer deposition. TheHDPE prefer- [212] entially adsorbs onto the Pt, narrowing the product distribu- opposed to random scission in polyolefins. tion without over cracking the products.[214] Asimpler, yet effective idea is nanocrystalline HZSM-5 which exhibits an 2.5.3. Hydrocracking and other Reactive Gasses external surface area of up to 20%ofthe total surface area, [215] helping to overcome diffusion limitations. Catalyst proper- Alternatives to N2 as apyrolysis atmosphere (e.g. H2)can ties and their influence on the reaction products are well be used to tune product distribution. Hydrocracking is summarized in the literature.[212] Yetthe search for acatalyst apromising process as it reduces coke formation through that is economical, selective,stable,and active enough to radical capping of the coke precursors and also operates at allow for industrial implementation is still ongoing. reduced reaction temperatures.These factors,inturn, prolong [212,217] the catalyst lifetime. Theaddition of H2 also creates 2.5.2. Reaction Mechanisms amore highly saturated product (alkanes rather than alkenes) which can then be cracked using established steam-cracker With afocus on polyolefins,thermal pyrolysis proceeds technology to form monomers for PP and PE synthesis.In through afree-radical mechanism whereas catalytic pyrolysis addition to hydrogen and nitrogen, helium, argon, ethylene, [218] [219] proceeds through acombination of free-radical and carbo- propylene, and CO2 have also been investigated. The cation mechanisms.Subsequent reaction steps,inter/intra- use of areactive gas reduces the coke formation and

molecular hydrogen transfer and b-scission, have different influences product yield. Pyrolysis in aCO2 rich atmosphere, activation energy barriers and the prominence of one path- for example,reduced the tar formation in PVC pyrolysis and way over another can vary widely with small changes in reduced the acidity of pyrolytic oil for PET.[220, 221] It was

temperature.Insight on the different pathways helps explain reported by Akah et al. that H2 also helps deal with any why product distribution changes with reaction conditions as heteroatoms (e.g. Br and Cl) in the waste plastic,[222] although seen in Figure 6. Thekey points are: handling of the generated acids may represent achallenge 1) Forpolyolefins,initiation reactions are believed to occur (e.g. with regard to reactor lifetime). through polymer chain imperfections rather than cracking of C Cbonds.[212] 2.5.4. Future Reactor Design À 2) Therate of random scission increases with the Mw of the

polymer chain resulting in atightening of the Mw Thechoice of reactor is important for plastic-waste distribution of the unreacted polymer through the reac- depolymerization. Forexample,afluidized bed reactor tion.[212] offers shorter reaction residence times,hence minimizing 3) b-scission requires higher energy but produces shorter secondary reactions and side product formation. Areactor, products than random scission in thermal pyrolysis,help- such as ascrew kiln, with longer residence times would

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Figure 6. With an increase in the numberoffunctional groups and heteroatoms in the backbone of the polymer (top), the distribution of products and the pyrolytic mechanisms become less complicated.Process parameters provide ahigher degree of control over the product distribution for polyolefins,inthis example HDPE, although the ultimate monomer yield is lower than for PS and PMMA. Acommon theme of all pyrolysis is that an excessively high temperature leads to coke formation given that the residence time is long enough. Although, most reaction steps occur at lower temperatures, similar trends are observed for catalyticprocesses. BTX denotes benzene, toluene,and xylene.

typically be used for fuels and aromatics production. As of plastic. Microwave assisted pyrolysis has also been touted discussed in Section 2.5, the following reaction parameters as apromising option.[224] However,the low dielectric provide ahandle for obtaining higher selectivity to desired constant of plastics,requires microwave absorbers that can products: generate hotspots.Plasma assisted pyrolysis facilitates rapid 1) Heating rate heating to high temperatures,leading to reaction steps not 2) Temperature (distribution) of reaction typically observed in conventional pyrolysis and as aresult 3) Solid/gaseous contact time achieves approximately double the monomer recovery 4) Gaseous residence time rates.[194]

These factors,inaddition to the challenges of physically handling the waste,should be considered carefully when 3. Concluding Remarks and Outlook selecting areactor design. In this context, aconical spouted 4 1 bed reactor facilitates high heating rates (10 8CsÀ )resulting Plastic waste has developed into apressing problem and in high monomer recovery rates from PS.[207] Inspiration can society is approaching acritical turning point. From an also be drawn from biomass pyrolysis using agas-solid vortex industrial level, several start-ups have been founded, and reactor[223] to overcome the heat and mass transfer limitations major polymer manufacturers are promising or taking action.

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Thecoming years will tell whether areal change can be improve current recyclingrates.The LCA analysis provides achieved in plastic recycling. In this context, chemical an initial ranking of the various technologies in terms of their

recycling will play an important role as plastic use can only potential to avoid CO2 emission. From this data we can be decreased to alimited extent and better package design, conclude that polymers should be kept intact as much as that is,byavoiding composite and multilayer materials,can possible during recycling,asthis will reduce overall energy only facilitate recycling, while not eliminating the plastic demand. This way leads to less energy required to break and waste problem. Reuse is only possible in very limited cases, reform chemical bonds.This is why dissolution/precipitation

while mechanical recycling leads to lower quality materials scores very highly in CO2 avoidance.The main problem for and cannot be applied to all plastic materials.However,any dissolution/precipitation is the use of hazardous organic plastic recycling depends on the improvement of cleaning and solvents,but in the CreaSolv Process,for example,these sorting, for example,bysmart and automated sensory are fully recycled. Thesecond-best option is to produce ahigh systems.Anexample for this is the most recycled polymer, purity stream of monomers,because they can directly be PET from bottles,for which efficient collection systems exist repolymerized without the need for an additional process in some countries.There is aclear mismatch in the purity of step.Monomer recovery is typically the highest in solvolysis, researched ideal plastics (mixtures) and available real-life but it can only be applied to polyesters and polyamides unless plastic waste-streams,which hampers commercialization. emerging techniques,such as ionic liquids or supercritical In this Review,wehave presented the chemical processes fluids,are used. Traditional pyrolysis is the least preferred of solvolysis,dissolution/precipitation, and pyrolysis as well as chemical recyclingoption in terms of LCA,but advances in novel ways of performing them, such as supercritical fluids, catalyst and reactor design could lead to pyrolysis that microwave reactors,mechanochemistry and biotechnology. directly yields monomers.Inaddition, it was demonstrated Owing to their respective limitations and advantages,amix of that emerging technologies,such as plasma-assisted pyrolysis, different chemical technologies will likely be needed to can produce monomers in very high yields.And even the

Table 3: Abbreviations Abbreviation Full name Synonyms/IUPAC ABS Acrylonitrile butadiene styrene BFR Brominated flame retardants BHET Bis(2-hydroxyethyl)-terephthalate Bis(2-hydroxyethyl)-terephthalate BTX Benzene, toluene,xylene DMT Dimethyl terephthalatedimethyl benzene-1,4-dicarboxylate EG Ethylene glycol Mono ethylene glycol (MEG), ethane-1,2-diol EU EuropeanUnion EoL End of life HBCD Hexabromocyclododecane 1,2,5,6,9,10-Hexabromocyclododecane HDPE High density polyethylene HIPS High Impact Polystyrene IL Ionic liquid LCA Life cycle analysis LDPE Low density polyethylene

Mw Molecular weight MHET Mono-(2-hydroxyethyl)terephthalate MHETase Mono-(2-hydroxyethyl)terephthalate-digestingenzyme MPW Municipalplastic waste PA Polyamide Nylon, Perlon PE Polyethylene PEF polyethylene-2,5-furandicarboxylate: PET alternative derived from bio-2,5-furandicarboxylic acid PET Polyethylene terephthalate PETase Polyethylene terephthalate-digesting enzyme PLA polylactic acid PMMA Polymethylmethacrylate Acrylic PP Polypropylene PP-GF Glass fiber reinforced polypropylene PS Polystyrene PSW Plastic solid waste PU, PUR Polyurethane PVC TPA Terephthalic acid Benzene-1,4-dicarboxylic acid, PTA(Purified Terephthalic Acid) TRL Technology Readiness Level UPR Unsaturated polyester resin WEEE Waste electric and electronicequipment wt%Percentage based on weight

15418 www.angewandte.org  2020 The Authors. Published by Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim Angew.Chem. Int. Ed. 2020, 59,15402 –15423 Angewandte Reviews Chemie more-traditional pyrolysis processes can be very useful when be tuned to high BTX or to high monomer recovery.For integrated smartly into existing refinery infrastructure and is, example,Seo et al. improved the weight fraction of aromatics according to our LCA,still advantageous over incineration in the oil product from HDPE from < 1% to 59%with with energy recovery.However, PVC and PU as well as PET azeolite ZSM-5 catalyst at 4508C.[200] In that respect further and PA should be avoided in pyrolysis or the plastic stream improvements also have to be made regarding plastic/catalyst has to be pre-treated, if the pyrolysis oil is to be fed to contact. Some proposed solutions are dissolving plastic, for asteam-cracker. example,inthe recycled vacuum gas oil fraction or in crude Another option is to produce other high value products oil prior to feeding to the pyrolysis reactor, using supercritical from polymers (i.e.BioCellection), which provides agood fluids or mechanochemical conversion. Amore even heat business case.From pyrolysis this could be avery pure stream distribution also helps to achieve higher yields of monomers of BTX (benzene,toluene,xylene). Diesel and gasoline are or other desired products.This can be achieved in microwave another option with alower market price.Solvolytic routes reactors,with supercritical fluids or by mechanochemical provide fine chemicals with avery high market price.Tuning conversion. solvolysis as well as pyrolysis processes towards value-added In other words,the future is bright for chemists and products also requires the use of stable and selective catalysts. chemical engineers to find new and improved processes for We conclude that the development of new or improved chemical recycling of awide variety of commonly used plastic catalysts,which not only very active and selective,but also materials,and much progress can be expected in the years to stable,isavery important factor in improving current come.Onthe other hand, these scientific and technological solvolysis and pyrolysis processes.Inpyrolysis products can developments will have to go hand in hand with better policy

Table 4: Process descriptions Process name Description Mechanical recycling Physical treatment of the plastic to achieve aconsumer product from plastic waste. The most (also:secondary recycling) common mechanicalrecycling process involvesmelting and re-extruding the plastic.

Chemical recycling Instead of merely physically transforming the shape and macroscopic properties of the plastic, (also:tertiary recycling, feedstock recycling) chemical changes are made through breaking bonds. Often the goal is to depolymerize the polymers into monomers. These can be used to synthesize new polymers, but other chemical building blocks can result as well. Feedstockrecycling is used to describe the recycling back to feedstocksused to make new polymers that is either monomers directly or acrude oil resemblingproduct that can be fed to steam-crackers to produce monomers.

Depolymerization Breaking the bonds of the polymers to form monomers or oligomers. Often other side products form as well due to side reactions or interactionwith areactive medium present during depolymerization.

Thermochemical routes Includes all processes (i.e. pyrolysis, hydropyrolysis, gasification) that break polymer bonds solely

through the input of thermal energy.This can be achieved under inert (i.e. N2)orreactive (i.e. H2 or

O2)atmosphere. These processes are most widely applied to polyolefins, but also studied for PS, PET, PMMA and impurities of other polymers.

Pyrolysis During pyrolysis (-lysis, Greek for dissociation) the chemical bonds of plastic are broken due to

(also:thermolysis,thermal cracking, catalytic thermal energy.The plastic is heated under inert atmosphere (i.e. N2)until permanentgasses, liquids cracking, liquefaction) and waxes are formed. This process usually yields avery mixed hydrocarbonstream. This process is also denotedcatalytic cracking or thermal cracking depending on whether acatalyst is used. Liquefaction refers to pyrolysis or hydropyrolysisunder pressure.

Hydropyrolysis Thermalbreak-down of plastic under H atmosphere. More specifically,hydrogenolysis refers to C C 2 À (also:hydrogenolysis, hydrocracking) bond cleavage followed by hydrogenation on amonofunctional metal catalyst. Hydrocracking refers to the same process on amonofunctional acid catalyst or on abifunctional catalyst comprising ametal and acid site (bifunctional hydrocracking).

Solvolysis Solvolysis is applicabletopolymers with heteroatomsintheir backbone and cannot be used to break C Cbonds. The solvolysis processes are named after the cleavage agent used and include hydrolysis, À alcoholysis (glycolysis and methanolysis), phosphorolysis, ammonolysis and aminolysis. Ether,ester and acid amide bonds can be cleaved this way.

Dissolution/precipitationInthis process aplastic containing additives and impuritiesofother polymers or materials is dissolved. Asolvent is chosen to selectivelydissolve the desired polymer.Unwanted additives are filtered out and the desired polymer is precipitated.Strictly speaking dissolution/precipitation is not achemical recycling process as usually no bonds are cleaved. However,since chemical fundamental knowledge is needed to understand the solvent/polymer interaction, solvent design and solvent recovery this process is covered in this perspectiveand is often considered chemical recycling.

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